The present invention relates to the control and synchronization of clocks used for driving counters to generate control pulses.
Conventional methods include driving a digital counter with a crystal-controlled clock. The counter is typically initialized with a value dependent on the base crystal frequency and the desired pulse rate. The counter then counts down by one each time it receives a pulse from the crystal. When the counter reaches zero, it produces an output pulse, resets to the initial count, and starts a new count down cycle. The resolution in the output rate of conventional counters is limited by the pulse frequency of the crystal. For example, a ten (10) megahertz crystal produces a resolution of 0.1 microseconds. For an application requiring a controllable output pulse rate of one (1) pulse every 250 microseconds, the resolution is 1 part in 2500. Some applications, however, such as applications that require close synchronization of data sampling at multiple locations, require a higher resolution.
Many power system monitoring, protection, and control functions could be performed more efficiently and accurately if power system digital measurements at multiple locations were synchronized. Generally such measurements are only somewhat synchronized because of difficulty in accurately synchronizing sampling clocks physically separated by large distances. Conventional uses of digital communications to synchronize sampling clocks at remote locations have accuracies limited by uncertainties in the message delivery time. In particular, digital communications can have different delays in different directions between a pair of locations which lead to an error in clock synchronization.
In addition to being important for multi-terminal transmission lines, time synchronization is important in many other applications such as power relays, determinations of sequences of events, economic power dispatch, and any other situation requiring synchronization of clocks.